The catalyzed carbonylation reaction of methanol with carbon monoxide provides an important synthetic route to acetic acid, and is significant as a C.sub.1 conversion process for producing bulk chemicals. As the third largest end use of synthetic methanol, acetic acid production consumes large quantities of methanol and carbon monoxide, one or both of which may be obtained from synthesis gas.
Methanol is the largest volume commodity chemical derived from C.sub.1 conversion processes, with 1993 production exceeding 2.1 million tonnes in Canada and 4.8 million tonnes in the U.S.A. Methanol serves as a building block for synthesizing many important chemicals. The largest end uses of synthetic methanol are the manufacture of methyl tertiary butyl ether (MTBE) via a catalytic addition reaction with isobutylene and the manufacture of formaldehyde via catalytic oxidation of methanol. The manufacture of acetic acid is the third largest end use of synthetic methanol and world production capacity for acetic acid is estimated at about 5.6 million tonnes annually, and about 60% of this capacity is based on the catalytic carbonylation reaction of methanol with carbon monoxide. In 1993, U.S. production of acetic acid exceeded 1.7 million tonnes. As such, the catalysed carbonylation reaction of methanol provides an important synthetic route to acetic acid and is significant as a C.sub.1 conversion process for producing bulk chemicals. Thus, it will be recognized that the economics of commercial acetic acid production are closely linked to those of methanol production.
There are a variety of commercial processes for producing acetic acid by methanol carbonylation. A common process begins with synthesis gas production and includes the steps of: synthesis gas production, synthesis gas to methanol conversion, carbon monoxide/hydrogen separation from synthesis gas, methanol carbonylation and acetic acid recovery.
Although the syngas route to acetic acid is currently an economically attractive process for large scale acetic acid production, several energy intensive intermediate steps add to the overall production costs. For instance, there is the cost of synthesis gas production, both in energy consumed and capital investment required. There is a further cost in hydrogen/carbon monoxide separation, which is again capital intensive and inefficient if there is no immediate use for the hydrogen other than its fuel value. Finally, there is capital and operating costs associated with decompression and compression of the process streams for the various synthesis steps.
Various researchers have described processes for, oxidation of methane to methanol. For instance, Hah et al. in U.S. Pat. No. 4,982,023 describe a process for the synthesis of methanol by the homogeneous direct partial oxidation of natural gas or other source of methane using a reactor in which the reactor space is filled with inert, refractory inorganic particles. Ramachandran et al., U.S. Pat. No. 5,278,319 describes a process for the production of hydrocarbon partial oxidation products in which the concentration of carbon monoxide and all parts of the system is maintained at a high level.
There is also much published information on the production of acetic acid from methanol. For instance European Patent Publication No. 0 526 974 A1, published Feb. 10, 1993 describes a process for preparing acetic acid which comprises contacting methanol with the carbon monoxide or a mixture of carbon monoxide with hydrogen of 2% by volume or less in the presence of a carbon-supported rhodium metal catalyst and methyl iodide promoter in vapor phase. In Catalysis Today 18 (1993) 325-354, Howard et al., have provided a far reaching discussion relating to the carbonylation of methanol to acetic acid, including a lengthy discussion of the catalysis that may be used in this reaction.
There remains a need for a simplified and less expensive route for converting methane to acetic acid which can avoid many of the expensive intermediate steps presently used.